In the area of wireless communication systems, the notion of using a relay channel for distributing a transmission from a source node to a destination node is a conventional idea, introduced as early as the 1970s; however, due to continued advancements in the capabilities of wireless systems over the decades, the notion has received renewed attention.
In this regard, cooperative communication through relay nodes has been proven to enhance the capacity of the communications channel, and has enabled a new architecture called virtual Multiple Inputs, Multiple Outputs (MIMO) arrays. In addition, data transmission through relay networks in wireless communication systems has been observed to have high spectral efficiency. As a result, for instance, the IEEE 802.11 Task Group and 802.16 Task Group are collaborating with respect to the standardization of certain relay network protocols.
However, there are number of limitations of current architectures for which improvement is generally desired. For instance, some conventionally known relay systems have assumed a full duplex relay. With a full duplex relay, the relay node is assumed to be able to transmit and receive at the same time and on the same channel.
Yet, such a full duplex relay system with perfect echo cancellation proves difficult to implement in practice in real systems. Instead, with real systems, sometimes orthogonal channels are used to isolate the transmitted and received signals at the relay node. Under such circumstances of orthogonal, or independent, transmit and receive channels, the relay node is referred to as a half duplex relay. Such half duplex relays have been considered in the context of frequency division, time division, code division and general orthogonal division. In particular, to exploit the frequency selectivity of broadband wireless systems, orthogonal frequency-division multiple-access (OFDMA) systems have been proposed for half duplex relay networks.
The problem remains, however, as a result of the constraints introduced by a half duplex relay node relative to a full duplex relay node, the use of the half duplex relay node may enhance or degrade the system throughput depending on the instantaneous channel states between the source and relay, the relay and destination as well as the source and destination. This is because when a half duplex relay node is used to transmit the message to the destination, the source node cannot transmit on the same time/frequency slot and therefore, there is overhead, or a cost, associated with using the relay.
As a consequence, if the relay is used at the wrong time, e.g., when the channel state of the relay-destination pair is considerably worse than the channel state of the source-destination pair, there is an observed degradation in the overall system throughput. Hence, the ability to dynamically adjust subcarrier resources allocated to an OFDMA half duplex relay node so that the relay is used only at the right time is desirable.
Conventional approaches that have sought a solution to this problem have dynamically scheduled the usage of the relay node in a centralized manner in which full knowledge of the channel states between any two nodes in the network is required. However, perfect knowledge of the channel states at various nodes is very difficult to obtain in a distributed network. In addition, superposition coding is assumed at the source and relay node, which is not scalable as the number of nodes in the relay networks increases in terms of both the processing complexity and the signaling overhead involved in collecting all the channel state information (CSI) for the nodes. Furthermore, with such conventional approaches, the presence or absence of relays is not transparent to the source node.
Accordingly, there exists a need for improved techniques for OFDMA wireless communications systems to transmit message(s) from a source node to a destination node by scheduling resources to one or more half-duplex relay nodes in an optimal manner.